7. Redirection

If a command is followed by &
and job control is not active,
then the default standard input
for the command is the empty file /dev/null.
Otherwise, the environment for the execution of a command contains the
file descriptors of the invoking shell as modified by
input/output specifications.

The following may appear anywhere in a simple command
or may precede or follow a complex command.
Expansion occurs before word or digit
is used except as noted below.
If the result of substitution on word
produces more than one filename,
redirection occurs for each
separate filename in turn.

<word

Open file word for reading as standard input.

<>word

Open file word for reading and writing as standard input.
If the file does not exist then it is created.

>word

Open file word for writing as standard output.
If the file does not exist then it is created.
If the file exists, and the CLOBBER option is unset,
this causes an error;
otherwise, it is truncated to zero length.

>|word

>!word

Same as >, except that the file is truncated to zero length
if it exists, even if CLOBBER is unset.

>>word

Open file word for writing in append mode as standard output.
If the file does not exist, and the CLOBBER
option is unset, this causes an error;
otherwise, the file is created.

>>|word

>>!word

Same as >>, except that the file is created if it does not
exist, even if CLOBBER is unset.

<<[-] word

The shell input is read up to a line that is the same as
word, or to an end-of-file.
No parameter expansion, command substitution or
filename generation is performed on word.
The resulting document, called a
here-document, becomes the standard input.

If any character of word is quoted with
single or double quotes or a ‘\’,
no interpretation is placed upon the characters of the document.
Otherwise, parameter and command substitution
occurs, ‘\’ followed by a newline is removed,
and ‘\’ must be used to quote the characters
‘\’, ‘$’, ‘‘’ and the first character of word.

Note that word itself does not undergo shell expansion. Backquotes
in word do not have their usual effect; instead they behave
similarly to double quotes, except that the backquotes themselves are
passed through unchanged. (This information is given for completeness
and it is not recommended that backquotes be used.) Quotes in the form
$’...’ have their standard effect of expanding backslashed
references to special characters.

If <<- is used, then all leading
tabs are stripped from word and from the document.

<<<word

Perform shell expansion on word and pass the result
to standard input. This is known as a here-string.
Compare the use of word in here-documents above, where word
does not undergo shell expansion.

<&number

>&number

The standard input/output is duplicated from file descriptor
number (see man page dup2(2)).

<& -

>& -

Close the standard input/output.

<& p

>& p

The input/output from/to the coprocess is moved to the standard input/output.

>&word

&>word

(Except where ‘>&word’ matches one of the above syntaxes;
‘&>’ can always be used to avoid this ambiguity.)
Redirects both standard output and standard error (file descriptor 2)
in the manner of ‘>word’.
Note that this does not have the same effect as ‘>word2>&1’
in the presence of multios (see the section below).

>&|word

>&!word

&>|word

&>!word

Redirects both standard output and standard error (file descriptor 2)
in the manner of ‘>|word’.

>>&word

&>>word

Redirects both standard output and standard error (file descriptor 2)
in the manner of ‘>>word’.

>>&|word

>>&!word

&>>|word

&>>!word

Redirects both standard output and standard error (file descriptor 2)
in the manner of ‘>>|word’.

If one of the above is preceded by a digit, then the file
descriptor referred to is that specified by the digit
instead of the default 0 or 1.
The order in which redirections are specified is significant.
The shell evaluates each redirection in terms of the
(file descriptor, file)
association at the time of evaluation.
For example:

... 1>fname2>&1

first associates file descriptor 1 with file fname.
It then associates file descriptor 2 with the file associated with file
descriptor 1 (that is, fname).
If the order of redirections were reversed,
file descriptor 2 would be associated
with the terminal (assuming file descriptor 1 had been)
and then file descriptor 1 would be associated with file fname.

The various forms of process substitution, ‘<(list)’,
and ‘=(list())’ for input and
‘>(list)’ for output, are often used together with
redirection. For example, if word in an output redirection is of the
form ‘>(list)’ then the output is piped to the
command represented by list. See
Process Substitution.

7.1 Opening file descriptors using parameters

When the shell is parsing arguments to a command, and the shell option
IGNORE_BRACES is not set, a different form of redirection is allowed:
instead of a digit before the operator there is a valid shell identifier
enclosed in braces. The shell will open a new file descriptor that
is guaranteed to be at least 10 and set the parameter named by the
identifier to the file descriptor opened. No whitespace is allowed
between the closing brace and the redirection character. For example:

... {myfd}>&1

This opens a new file descriptor that is a duplicate of file descriptor
1 and sets the parameter myfd to the number of the file descriptor,
which will be at least 10. The new file descriptor can be written to using
the syntax >&$myfd.

The syntax {varid}>&-, for example {myfd}>&-, may be used
to close a file descriptor opened in this fashion. Note that the
parameter given by varid must previously be set to a file descriptor
in this case.

It is an error to open or close a file descriptor in this fashion when the
parameter is readonly. However, it is not an error to read or write a file
descriptor using <&$param or >&$param if param is
readonly.

If the option CLOBBER is unset, it is an error to open a file
descriptor using a parameter that is already set to an open file descriptor
previously allocated by this mechanism. Unsetting the parameter before
using it for allocating a file descriptor avoids the error.

Note that this mechanism merely allocates or closes a file descriptor; it
does not perform any redirections from or to it. It is usually convenient
to allocate a file descriptor prior to use as an argument to exec.
The syntax does not in any case work when used around complex commands
such as parenthesised subshells or loops, where the opening brace is
interpreted as part of a command list to be executed in the current shell.

The following shows a typical sequence of allocation, use, and closing of a
file descriptor:

Note that the expansion of the variable in the expression >&$myfd
occurs at the point the redirection is opened. This is after the expansion
of command arguments and after any redirections to the left on the command
line have been processed.

7.2 Multios

If the user tries to open a file descriptor for writing more than once,
the shell opens the file descriptor as a pipe to a process that copies
its input to all the specified outputs, similar to tee,
provided the MULTIOS option is set, as it is by default. Thus:

date >foo >bar

writes the date to two files, named ‘foo’ and ‘bar’.
Note that a pipe is an implicit redirection; thus

date >foo | cat

writes the date to the file ‘foo’, and also pipes it to cat.

If the MULTIOS
option is set, the word after a redirection operator is also subjected
to filename generation (globbing). Thus

: > *

will truncate all files in the current directory,
assuming there’s at least one. (Without the MULTIOS
option, it would create an empty file called ‘*’.)
Similarly, you can do

echo exit 0 >> *.sh

If the user tries to open a file descriptor for reading more than once,
the shell opens the file descriptor as a pipe to a process that copies
all the specified inputs to its output in the order
specified, similar to cat,
provided the MULTIOS option is set. Thus

sort <foo <fubar

or even

sort <f{oo,ubar}

is equivalent to ‘cat foo fubar | sort’.

Expansion of the redirection argument occurs at the point the redirection
is opened, at the point described above for the expansion of the variable
in >&$myfd.

Note that a pipe is an implicit redirection; thus

cat bar | sort <foo

is equivalent to ‘cat bar foo | sort’ (note the order of the inputs).

If the MULTIOS option is unset,
each redirection replaces the previous redirection for that file descriptor.
However, all files redirected to are actually opened, so

echo foo > bar > baz

when MULTIOS is unset will truncate bar, and write ‘foo’ into baz.

There is a problem when an output multio is attached to an external
program. A simple example shows this:

cat file >file1 >file2
cat file1 file2

Here, it is possible that the second ‘cat’ will not display the full
contents of file1 and file2 (i.e. the original contents of
file repeated twice).

The reason for this is that the multios are spawned after the cat
process is forked from the parent shell, so the parent shell does not
wait for the multios to finish writing data. This means the command as
shown can exit before file1 and file2 are completely written.
As a workaround, it is possible to run the cat process as part of a
job in the current shell:

7.3 Redirections with no command

When a simple command consists of one or more redirection operators
and zero or more parameter assignments, but no command name, zsh can
behave in several ways.

If the parameter NULLCMD is not set or the option CSH_NULLCMD is
set, an error is caused. This is the csh behavior and CSH_NULLCMD
is set by default when emulating csh.

If the option SH_NULLCMD is set, the builtin ‘:’ is inserted as a
command with the given redirections. This is the default when emulating
sh or ksh.

Otherwise, if the parameter NULLCMD is set, its value will be used as a
command with the given redirections. If both NULLCMD and
READNULLCMD are set, then the value of the latter will be used instead
of that of the former when the redirection is an input. The default for
NULLCMD is ‘cat’ and for READNULLCMD is ‘more’. Thus

< file

shows the contents of file on standard output, with paging if that is a
terminal. NULLCMD and READNULLCMD may refer to shell functions.